What's Wrong With This Cover?

 Going beyond the normal 'fill-the-album-spaces' mode of stamp collecting, I also enjoy an aspect of the hobby termed as 'postal history'.

Postal history entails stamps that are on-cover, i.e., envelopes and postcards. Each such piece has its own small story to tell, since the cover includes the name of the sender, recipient, a postmark cancellation which shows the date and postal facility through which the item was sent, as well as other interesting notations or markings that may have been applied during the piece's journey through the mail stream.

The cover shown in this post has such an interesting story... 

The letter is franked with a Christmas Seal rather than a regular postage stamp!

Normally, such a piece would be detected by a mail clerk and returned back to the sender with a note of  'insufficient postage'.

In this case, the clerk may have missed the error, perhaps due to the Christmas rush of processing high volumes of mail.

I like to think that the sender might have been a child who wasn't aware of the difference between a Christmas seal and a postage stamp. After all, they look about the same.  I envision a postal clerk recognizing the letter for what it was, smiling knowingly, applying the cancellation, and sending it on its way....